There are two pigments generally regarded as contributing to the color of red meat, myoglobin and hemoglobin. Both of these pigments are purplish in color with hemoglobin in the blood and myoglobin in the tissue. Since an animal such as a cow, pig, sheep, goat, or chicken is typically bled when it is slaughtered, most of the color pigment remaining in the carcass is myoglobin. Raw red meat that is freshly cut is characteristically purplish in color, mostly as a result of the myoglobin presence. Exposure of the myoglobin to an adequate supply of oxygen results in the formation of oxymyoglobin which gives the desirable red color of the meat. The same type of reaction and change in color occurs with hemoglobin, so for whole fish which has not been bled to the extent as a cow, pig, sheep, goat, or chicken, exposure of the myoglobin and hemoglobin results in the formation of oxymyoglobin and oxyhemoglobin.
With an inadequate supply of oxygen, myoglobin and hemoglobin are converted to metmyoglobin and methemoglobin. Metmyoglobin and methemoglobin are brownish-grey or brownish-blue in color, i.e., the color commonly seen in meat or fish surfaces that are old. Some transparent food packaging films are relatively impermeable to oxygen and the red color of the oxymyoglobin and oxyhemoglobin gradually changes to brownish-grey or brownish-blue and the meat or fish packaged in them becomes unsalable, sometimes as rapid as two to four days after packaging.
Carbon monoxide (CO) acts as a color stabilizer in red meat by binding with myoglobin and hemoglobin creating carboxymyglobin and carboxyhemoglobin, respectively, which are cherry-red in color. CO is an odorless, colorless gas, yet very toxic to humans and animals. In the United States, the use of CO (at amounts up to 0.4% or 4000 ppm) has been approved for packaging of fresh meat and retail packaging applications.
There is an increasing demand from food companies to use CO for stabilization of meat color. However, industrial gas companies may decline to participate in this market due to the safety and liability concerns arising from the handling and distribution of CO in bulk quantities. This is because the relatively pure CO used for blending with other gases poses a substantial asphyxiation risk to operators during the blending process.
Plasma is an at least partially ionized gas composed of ions, electrons, and depending upon the degree of ionization, neutral particles. Plasma is a state of matter distinguishable from solids, liquids, gases, and supercritical fluids. Compared to gas in its natural state, plasma contains free charged particles, electrons and ions, although it is overall electrically neutral. Different types of man-made plasmas can be categorized based upon pressure: vacuum, low pressure, and atmospheric pressure. They can also be categorized based upon whether thermal equilibrium exists between the ions and electrons. In thermal plasmas, the temperature of the heavy ions is equal to the temperature of the electrons.
Non-thermal plasma (also sometimes referred to as cold plasma or non-equilibrium plasma) is in general any plasma which is not in thermodynamic equilibrium, either because the ion temperature is different from the electron temperature, or because the velocity distribution of one of the species does not follow a Maxwell-Boltzmann distribution. As opposed to thermal plasmas where all particles of the medium (neutral molecules, atoms and radicals, ions and electrons have roughly the same energy distribution (meaning a common temperature), in non-thermal plasma electrons have a much higher average energy than heavy species. A limit to such a situation is with the so called cold plasma, corresponding to gas temperature (meaning average energy of the heavy species) is close to ambient. However, some types of plasma may exist that are non-thermal but not cold, with heavy species temperature less than one order of magnitude below the electron temperature. In general, such plasmas are sustained by electrical discharges in a gas close to atmospheric pressure and must be distinguished from other mature, industrially applied plasma technologies like welding, cutting and thermal spraying.
In non-thermal plasma, the free electrons are excited through acceleration by an electric field created by an external source of excitation. In parallel to this acceleration, the electrons undergo random frequent elastic collisions with the molecules and ions, also called heavy particles. Thus electrons continuously gain energy over time in the form of a disordered motion that has similarities with thermal agitation, but is “forced” by electrical energy input and much more intense. The average electron energy corresponds to an equivalent temperature of the order of tens of thousands of degrees. The average energy of electrons is much higher than the heavy particles. If the collisions are not too frequent, in the case of a rarefied gas, for example, they transfer only little energy to the heavy particles and preserve their movement of thermal agitation corresponding to the ambient. If the electrons acquire a very high “temperature” (i.e. average agitation energy) of the order of 104 K they produce inelastic collisions with the heavy particles that produces excitation (in terms of electronic level or vibrational quantified level), ionization (that constantly replenishes the population of electrons and ions to sustain a steady plasma), or dissociation into smaller fragments, atoms and radicals. The excited particles conceal very high “chemical energy” and can be reactive enough to produce surface treatments to a material, without the need to heat the material.
While atmospheric pressure plasma may be produced by several different methods, common ones include corona discharge, dielectric barrier discharge (DBD), and capacitive discharge. A corona discharge is a non-thermal discharge produced through application of high voltage to relatively sharp surfaces of electrode tips. They are commonly used in ozone generators and particle precipitators. DBD is a non-thermal discharge produced through application of high voltages across a small gap between electrodes, but in contrast to corona discharge, DBD requires a dielectric material to prevent the plasma discharge from becoming an electrical arc. DBD is commonly used for surface functionalization of webs and films often for achieving greater adherence of inks, paints, and glues. Capacitive discharge is non-thermal plasma generated through application of radiofrequency (RF) power (e.g., 13.56 MHz) to a powered electrode spaced apart at short distance from a ground electrode. Capacitive discharges are commonly stabilized with a noble gas such as helium or argon.
Corona discharge has long been used for ozone sanitization or sterilization of a wide variety of products. Ozone in such applications is typically generated remote from the chamber in which the product is treated. However, several have suggested the use of a combined combination ozone generation and product treatment chamber wherein a product is placed in a chamber and ozone is produced through corona discharge within the chamber.
In recent years, atmospheric non-thermal plasma has been investigated for sanitization of food products. Bacterial contamination in food products such as packaged spinach has been achieved through treated with ozone generated from non-thermal plasma in either air or oxygen. Paul A. Klockow, Kevin M. Keener, “Safety and quality assessment of packaged spinach treated with a novel ozone-generation system”, LWT—Food Science and Technology (2009), doi:10.1016/j.lwt.2009.02.011. Ozone is a strong oxidizer and is known to have a deleterious effect upon color stability in food due to its pigment bleaching effect. Microbial surface inactivation through microwave plasmas has also been suggested with the use of helium or krypton to reduce the temperature of the plasma. N. Tran, M. Amidi and P. Sanguansri, “Cool plasmas for large scale chemical-free microbial inactivation”, Food Australia 60 (8), pp. 344-347.
These attempts to produce plasmas for surface microbial inactivation have either used substantial amounts of oxygen (such as air or oxygen) or have not shown any improvement of color stability.
Thus, there is a need for obtaining the benefits available from non-thermal plasma processing of food products without deleteriously impacting the fresh color of the food product.
As mentioned above, conventional CO-containing MAP gas production poses a substantial asphyxiation risk to operators during the blending process.
Thus, there is a need for a safer way to provide CO for meat or seafood color stabilization.